Rainshadow by Milly Philips

Rainshadow is everything Flora has ever dreamed of…
a cliffside estate on a remote, rain-soaked island in the Pacific Northwest. She knows she belongs there, even though she has no money and no claim. She knows she belongs there, even if she hates the new lady of the manor, cruel and dismissive Sylvia.
Sylvia’s husband Ethan is young, mysterious, elegant, and beguiling. The horses, spirited and beautiful, are afraid of something that Flora can’t see. There are warnings all around her- there is evil at Rainshadow, an evil that threatens Flora, and maybe Ethan. She should run away, far, far away, but Flora belongs at Rainshadow.
And maybe, if she can save Rainshadow from evil, she can have everything.